Working with pictures

I have many pictures in grey. I want to be able to turn them to black & white pictures(no grey, just black & white). My ideia is that any color that is more near to black would turn black, and any color that is more near of white would turn white. Now I don't have any kind of clue of which classes to look for in GDI+. Anyone can point me a finger here? :afrog:

Something like this:

VB Code:

Dim objImage As Bitmap
        objImage = New Bitmap("C:\Temp\Test.jpg")
        Dim i As Integer
        Dim j As Integer
 
        Dim objOutput As New Bitmap(objImage.Width, objImage.Height, Imaging.PixelFormat.Format24bppRgb)
        For i = 0 To objImage.Width - 1
            For j = 0 To objImage.Height - 1
                'Since it is grayscale all 3 RGB values are the same, so we just check one
                If objImage.GetPixel(i, j).R < 128 Then
                    objOutput.SetPixel(i, j, Color.Black)
                Else
                    objOutput.SetPixel(i, j, Color.White)
                End If
            Next
        Next
 
        objOutput.Save("C:\Temp\Output.jpg", System.Drawing.Imaging.ImageFormat.Jpeg)
        MsgBox("Done")

That's what I thought at first, but that way surely would take many time for each image(well..actually the pictures are just 20x20) but they are A LOT and I'll have to be continuosly doing the process. I thought maybe there would be something along the matrixcolor classes or something that would be faster.

But if there's not, then thanks by the code

Have you seen this link:

http://www.bobpowell.net/onebit.htm

Looks like his way is very similar to mine.

I'm actually having problems puting the code to work as it seems I can't do SetPixel with the format of the picture I'm using.

Additional information: SetPixel is not supported for images with indexed pixel formats.How can I convert the pixel format to an affordable one?